This invention relates to a process and apparatus for converting pulp mill screen rejects and/or knots into usable pulp, and more particularly to a process and apparatus for refining and oxygen delignifying pulp mill screen rejects and/or knots to produce a bleachable grade pulp.
Handling and disposal of screen rejects and knots from pulp mills have been a problem for many years. Screen rejects, or fine screen rejects as they are sometimes called, are coarse particles too large to pass through a fine screen of a given size. Screen rejects include shives, slivers, chops, and bark. Knots, or coarse screen or knotter rejects as they are sometimes called, are also oversized particles and include the knot part of the wood, partially cooked and uncooked good wood chips, large shives and fiber bundles too large to pass through a 1/4 inch screen perforation.
For mills utilizing batch digesters, the knots and screen rejects can be recycled back into the digester and recooked. However, the resulting pulp is of low yield, and the rejects consume cooking chemicals and sometimes create channeling of pulping liquor within the digester. Moreover, the increasingly widespread use of continuous digesters has rendered recycle of the screen rejects impractical since they will eventually plug the liquor extraction screens used in continuous processes.
In pulp mills making unbleached grades of pulp, the screen rejects can be mechanically refined and be put back in the main pulp streams. However, in pulp mills making bleached grades of pulp, the screen rejects have often been heretofore unusable; these rejects must be removed from the process, dewatered, and then burned or hauled to a dump site. In the past, attempts at treatment by use of a separate bleaching step have not proved successful. In some cases in mills making unbleached grades, the knots have been mechanically refined and used in coarse grades of paper and board. However, such usage is very limited and may often not be economically viable. Traditionally, the knots are recooked in the digester. In some cases they are dewatered and dumped or burned.
Recently, however, processes have been reported in which screen rejects and knots have been delignified using oxygen to a bleachable level. Kirschner, Paper Trade Journal, p. 32 (Nov. 15, 1978), has reported the use of a low-consistency oxygen delignification process for draft and sulfite screen rejects which produces a bleachable grade of pulp. Hasvold, 1978 International Sulfite Conference, Montreal, Canada (Sept. 13, 1978), has reported an oxygen process which delignifies sulfite knots at a 25% pulp consistency.
However, both of these processes suffer from several disadvantages. Low-consistency operation requires a large reactor volume to maintain an acceptable retention time for the pulp. Operating at low consistency also produces large power demands for pumping large volumes of pulp and a high steam usage to heat the pulp in the reactor. Additionally, the low concentration of dissolved lignins in the pulp liquor increases evaporation costs for chemical recovery processes. Operation at 25% consistency, on the other hand, usually requires special dewatering equipment to attain the higher consistency. It is also known that high consistency operation of an oxygen delignification system can result in overheating of the pulp due to the exothermic delignification reaction, as well as pulp degradation, and even combustion of the pulp.
As can be seen, the need exists in the art for an efficient and economical process for utilizing pulp mill screen rejects and knots and converting them into bleachable grade pulp.